The growth of the demand for data from the Internet and other networks increases the demand for higher speeds in transmission and delivery systems, especially at the last mile. Nowadays the only feasible and cost-effective way to comply with actual requirements is using optical networks. Thus, some topologies based on Passive Optical Networks, PONS are being used as a solution. Thereat, the deployment of the networks called of Fibre To The “x” (FTTx, where x could be home, building, curb, etc) is an ordinary reality increasing the reach of these PONS. However, with the increasing number of spread fibres and cables, the probability of fault at some point of the PON also increases.
In order to ensure the operation within the acceptable requirements demanded by the current standards organizations and governments, the supervision or monitoring of these deployed networks should be done with more demanding requirements than traditional supervision or monitoring systems, in spite of the lack of standardized methods or tools, except for some general recommendations in ITU-T L-series. On the other hand, the capital and operational expenses with their infrastructure should be adequate to equalize investment and profit. Thus, the operators need to provide solutions to monitor their FTTx networks such that they are relatively inexpensive.
Basically, the technical requirements are higher resolution than traditional monitoring systems based in Optical Time Domain Reflectometry, OTDR, in order to localise faults with reasonable accuracy, and enough dynamic range to monitor split ratios up to 1:128 that could be used in the remote nodes and differential distances up to 60 Km.
In order to meet the technical requirements, some examples have been presented as a possible solution, but most of them are very far from feasible, either due to the high cost or the complex management of the markers or fibre lengths in order to identify each optical network unit (ONU) or terminal (ONT).
The main purpose of a fault monitoring system is the detection and location of failures in PON network fibre links, and many techniques have already been developed and implemented with this purpose. The main challenge of the failure monitoring system is the compromise between long range and high precision, which requires a high dynamic range.
OTDR-based systems with auxiliary data processing are widely employed. However, small losses can be neglected, unless a very large number of measurements is performed in order to obtain an average signal with a very low noise level, which requires a long sampling time. There is also a great challenge to identify in which branch, or network branch, the failure is situated. By a branch is meant the fibre link connecting a splitter, or Remote Node, RN, with an ONU or an ONT.
In order to try to identify in which of the network branches the fault is occurring and to determine its precise location, some techniques use reflectors with distinct reflection coefficients (mostly low reflection coefficients) installed at many points along the link. However, there are some disadvantages associated with such a solution for example low precision, high cost and complexity with respect to the installation of the reflectors along many points of the PON.
One solution is a set up with T-OTDR (tunable OTDR) and a Wavelength Division Multiplexer, WDM, as a by-pass to forward the T-OTDR signal to the branch to be monitored. However this technique requires the WDM to be installed within the RN. It adds extra costs and increases the power loss to the OTDR signal and to a data signal as well. Another disadvantage of this technique is the inability to monitor and detect any fault into the by-pass system (WDM, Arrayed Wavelength Grating, AWG, etc.). In case of a fault occurring inside the by-pass on the data connection, the OTDR signal will be by-passed showing a regular trace in the OTDR trace but the data signal will be interrupted.
Brillouin-based OTDR is another example of a solution for the identification and location of faults in fibre branches of the same length as the Brillouin frequencies are unique for all fibres. However, this system is rather complex and expensive, as special fibres must be installed and this system has not yet been shown to be able to monitor standard splitting ratios greater than 64.
Photon-counting OTDR features high dynamic range is still another example, but it shares the problems of standard OTDR concerning complex data processing and fault location in arms of the same length, which are problems yet to be solved. Techniques based on optical coding (OCDMA-inspired) are cost-attractive solutions, yet many aspects must be improved, such as data processing, power budget constraints and generalization to all PON topologies.